1. Technical Field
This invention relates generally to a system for controlling a switched reluctance (SR) motor, and more particularly, to a method for providing drive protection for an SR electric motor using a bootstrap gate drive circuit.
2. Discussion of the Related Art
Switched Reluctance Machines (SRMs) have been the subject of increased investigation due to their many advantages, which makes them suitable for use in a wide variety of situations. A SR machine operates on the basis of varying reluctance in its several magnetic circuits. In particular, such machines are generally doubly salient motors--that is, they have teeth or poles on both the stator and the rotor. The stator poles have windings which form machine phases of the motor. In a common configuration, stator windings on diametrically opposite poles are connected in series to form one machine phase.
When a machine phase is energized, the closest rotor pole pair is attracted towards the stator pole pair having the energized stator winding, thus minimizing the reluctance of the magnetic path. By energizing consecutive stator windings (i.e., machine phases) in succession, in a cyclical fashion, it is possible to develop torque, and thus rotation of the rotor in either a clockwise, or counter-clockwise direction. As further background, the inductance of a stator winding associated with the stator pole pair varies as a function of rotor position. Specifically, the inductance varies from a lower level, when a rotor pole is unaligned with a corresponding stator pole, to an upper or maximum level when the rotor pole and stator pole are in alignment. Thus, when a rotor pole rotates and sweeps past a stator pole, the inductance of the stator winding varies through lower-upper-lower inductance levels. This inductance-versus-rotor position characteristic is particularly relevant for controlled operation of the motor. Specifically, current flowing through the stator winding must be switched on (e.g., via power electronics) prior to (i.e., advanced), and maintained during the rising inductance period in order to develop a positive torque. Since positive phase current in the decreasing inductance interval produces a negative or breaking torque, the phase current must be switched off (e.g., by deenergizing the power electronics) before this interval occurs in order to avoid generating negative torque. Accordingly, rotor position sensing is an integral part of a closed-loop switched reluctance motor drive system so as to appropriately control torque generation.
Further, in a switched reluctance motor drive application, the power electronics (i.e., the semiconductor switches for energizing the stator windings) can potentially fail in the event of a prolonged, excessive load on the output of the motor. Therefore, there is a need to provide protection for the power electronics in the event of such an excessive, prolonged load, which is often unexpected (e.g., a pump connected to the output of the motor that seizes and the causes the motor to stall). The art has approached this problem by providing, in a typical configuration, a protection scheme which involves monitoring the current in the machine phases, and disabling the motor drive (i.e., disabling the power electronics) in the event of excessive current therethrough for a prolonged period of time. Although accomplishing, generally, the desired end of overload current protection, these prior art approaches are complicated, costly to implement (i.e., require extra components), as well as being prone to false shutdowns.
Accordingly, there is a need to provide an improved system for controlling a switched reluctance machine that minimizes or eliminates one or more of the problems set forth above.